The present invention relates in general to extruded plastic zipper structure of the reclosably interlockable type, having pull flange extensions and adapted to be secured in any preferred manner to the mouth end portions of bags or other packages; and is more particularly concerned with providing such zipper structure in a form especially suitable to be supplied in sectional or pre-cut strips for advantageous storage and handling, and convenient attachment to and between the bag mouth end portions.
In one manner of supplying bags with plastic zippers, the zippers have been extruded integrally with the bag material so that one of the complementary zipper strips is on one of the bag walls adjacent the mouth end of the bag and the other of the complementary zipper strips is on the other of the bag walls, the zipper strips being opposingly aligned for the interlocking purpose. Examples of such structures are found in U.S. Pat. Nos. 3,565,147, Re. 27,174, 3,827,472 and 4,665,552. Each of these patents also discloses ribbing for various purposes adjacent to the zipper profiles.
Formation of the zipper strips separate from and then attached or attachable to the bag making sheet material is found at various places in the art. By way of example, U.S. Pat. Nos. 4,528,224, 4,655,862, 4,673,383, 4,709,398 and 4,731,911 are referred to as disclosing feeding of zipper strips from rolls for application in whatever length desired to bag making or package making material.
U.S. Pat. No. 4,736,451 is referred to as disclosing an interesting example of preformed and attached zipper strip wherein the male profile has adjacent thereto a stabilizing post flange structure.
A major problem with rolled or spooled extruded plastic continuous zipper strips is that they developed curvature and camber. It may be noted that winding of any flexible plastic extrusion in whatever configuration inherently distorts straightness of the finished product which is visible when the strips are removed from the spool or reel. This is almost an inevitable result of the winding systems employed which, irrespective of taper, tension or torque winding require sufficient tension to be applied to the extruded product to ensure accurate placement of that product on the spool or reel. As a result, the extruded product is in a state of flux in that it has not achieved its final fully crystallized condition before being wound due to production speed requirements. Therefore, in the extruded wound product, malleable condition stresses and strains are set into the product. As the strips are wound, subsequent shrinkage takes place over a period of time. The shrinkage virtually ensures that irrespective of the type of winding or plastic material used, the strips will tighten down to an increased degree around the spool circumference. For example, if a five foot length of extruded plastic zipper strip is laid out on a floor, it will shrink at a known rate provided both ends of the product are not trapped. The same five feet of such zipper strip wound onto a spool and trapped on either end cannot shrink over a period of time. Therefore, the strip has locked-in stress and strain that cannot be relieved.
Locked-in stress and strain can be avoided by supplying the strips in unbent and untensioned cut lengths or sections. U.S. Pat. No. 4,691,373 mentions supplying such zipper strips in individual sections having zipper profiles and laterally extending web portions or flanges. This patent also has a disclosure of forming, cooling, and joining the zipper strips into reclosable unit sections. However, a major problem with the zipper strip sections as there shown is that due to the unsymmetrical cross-sectional configuration of the strips caused by the greater mass of the zipper profiles relative to the web flanges, much difficulty has been encountered in stacking or packing the strips one upon another.